Coordinated response between cysteine (Cys) metabolisms as up-stream and GSH-dependent antioxidant defense as down-stream thiol cascade was investigated in two genotypes of hardy legume grass pea (Lathyrus sativus L.), namely a glutathione (GSH)-deficient mutant (gshL-1) and BioL-212 genotype under (a) 40 μM As (sodium arsenate) and (b) As +1 mM BSO (L-buthionine-sulfoximine) treatments. As-treatment alone induced biosynthesis of Cys, GSH, and phytochelatins but partially stimulated GSH-dependent antioxidant defense in the mutant. GSH S-transferase activity was elevated but activities of GSH-dependent ascorbate (AsA)-GSH cycle enzyme including GSH-reductase remained low in the As-exposed mutant. This resulted in As-induced partial growth inhibition of the mutant. At As + BSO, GSH-synthesis was totally inhibited, leading to significantly low GSH-redox pool and steep decline in GSH-dependent antioxidant capacity of both the genotypes. However, Cys-degradation pathway was induced in gshL-1 mutant, resulting in significant accumulation of endogenous H2S. This was accompanied with stimulation of AsA-dependent antioxidant defense and catalases and regulation of Cys-synthesis, preventing over-accumulation of H2O2 and free Cys, respectively. No As-induced oxidative damage and growth inhibition were observed in As + BSO-treated mutant. Contrastingly, failure to induce entire defense cascade under BSO resulted in onset of As-induced oxidative stress in BioL-212 genotype. Results revealed that major metabolic reshuffling is imminent in both up-stream and down-stream thiol cascade during low GSH-redox, and H2S may confer As-tolerance through stimulation of GSH-independent antioxidant defense and maintaining Cys-homeostasis in grass pea mutant. © 2015, Botanical Society of Sao Paulo.